1. Field of the Invention
The present invention relates to polymeric blends of polyvinylchloride. More particularly, the present invention relates to flexible blends of polyvinylchloride and ethylene terpolymers that exhibit improved stability and processibility.
2. Brief Description of the Prior Art
Blends of polyvinylchloride with other polymeric materials are widely used in a variety of applications such as coatings and jacketings for wire and cable, roofing membranes, etc. In particular, blends of polyvinylchloride with ethylene copolymers such as terpolymers of ethylene, carbon monoxide and polymerizable esters such as vinyl acetate, methyl acrylate, butyl acrylate and the like are known.
In formulating polyvinylchloride (PVC) blends for use in such applications as wire and cable coating and jacketing, lubricants, generally consisting of one or more compounds, are incorporated into the blends to facilitate the processing of the blends by improving flow properties and stability of the polymer melt. The lubricants also minimize adherence of the molten polymer to machine surfaces during compounding and processing. A wide variety of such materials are commonly used as lubricants or lubricant packages. For example, fatty acids and their metal salts, paraffin wax, oxidized polyethylene wax, fatty acid esters, fatty acid amides, to name a few, have all been used in such PVC blends.
It is also common in flexible PVC formulations or blends, e.g., for wire and cable applications, to incorporate stabilizers to impart electrical insulation resistance and long-term thermal and processing stability. In particular, lead-based stabilizers such as lead phthalate are widely used in flexible PVC formulations. Such stabilizers, in addition to being cheaper than tin and barium/zinc-based stabilizers, provide superior performance as to electrical insulation resistance and long-term thermal and processing stability.
One problem associated with use of lead-based stabilizers in PVC blends containing ethylene terpolymers such as those discussed above is the tendency of the lead stabilizers to catalyze the cross-linking of the ethylene terpolymer. Such cross-linking impairs the processibility of the blends in that it results in large increases in viscosity during processing, e.g., extrusion, reducing the thermal stability and deleteriously affecting the flow properties of the polymer melt.